1. Field of the Invention
This invention relates to electric motors and in particular, to fractional horsepower universal motors as used in household appliances.
2. Background Art
Universal motors have a wound rotor with carbon brushes rubbing on a commutator to transfer electrical power to the rotor windings. In typical fractional horsepower universal motors, end brackets mounted to either end of the stator core support the rotor bearings. One end bracket also supports the cages for the brushes. Usually the end brackets are bent metal plates for good strength characteristics allowing a minimal axial length and providing an earthing function for the motor shaft via the bearings.
During operation of the motor, the brushes wear on the commutator generating considerable dust. This dust is conductive and builds up on the end bracket and around the brush cage assembly which is an insulating plastic body fixed to the end bracket and the brush slides directly in a hole in the plastic body for guiding the brush to the commutator. When sufficient dust has built up around the brush assembly, tracking may occur between the brush and the end bracket leading to an earth fault or insulation breakdown. One test, known as a hi-pot test, applies a 1000 volt potential difference between a motor terminal and the stator core or other non-live metallic part of the motor. Current is measured after 1 minute. If the current is more than 5 milliamps, the motor fails the test. For safety reasons, the motor must pass the hi-pot test at the end of its design life. Therefore, as design life increases, something must be done to avoid the build-up of the carbon dust resulting in hi-pot failure (insulation breakdown due to carbon dust build-up).
One prior art solution was to make the end bracket out of an insulating material. While this gave an immediate increase in life, it was still a problem as the carbon dust builds up all over the end bracket eventually allowing tracking. Also, as the plastic material was not as strong as the metallic material, larger end brackets were needed which added to the axial length of the motor.
Another prior art solution uses insulating sheet material, such as MYLAR, to form a dust barrier between the brush cage and the end bracket. This produced a marked increase in life but with improvements in bearings, dust build-up was still a limiting factor in the life of the motor as the dust collected on and under the MYLAR sheet.
Hence, there is a need for some way to avoid the build-up of carbon dust leading to insulation breakdown and thereby extending the useful life of a universal motor.